(CNN)US Secretary of State Nominee Rex Tillerson says that the country will preserve its "longstanding friendship" with the Philippines, despite the violent anti-drug campaign of leader Rodrigo Duterte.

During Tillerson's Senate Foreign Relations Committee confirmation hearing on Wednesday, Senator Marco Rubio asked Tillerson whether the killings in the Philippines were "conducive to human rights violations that we should be concerned about and condemning."

Tillerson said he'd need further "facts from the ground" before commenting.

Since Duterte took power in June, he's waged a bloody so-called "war on drugs," in which he's encouraged police and vigilantes to shoot drug dealers and users.

Human rights advocates have condemned the campaign, as has the Obama administration. However, Duterte said in a call last month that President-elect Donald Trump told him he was going about the drugs war "in the right way."

Since coming to power, Duterte has tested the country's relationship with the US -- at one point announcing a military and economic "separation" -- but then announcing he could work with the US now that Trump was in charge.